This invention relates generally to portable machines for fusing polyolefin pipe and more particularly concerns heaters used to bring the pipe surfaces to be joined to a molten state prior to fusion.
The end-to-end welding of polyolefin pipes is the subject of earlier U.S. Pat. No. 3,729,360 issued on Apr. 24, 1973 for a Portable Thermoplastic Pipe Fusion Apparatus, U.S. Pat. No. 4,352,708 issued on Oct. 5, 1982 for a Defined Force Fusion Machine for Joining Plastic Pipe and U.S. Pat. No. 5,814,182 issued Sep. 29, 1998 for a Self-Contained and Self-Propelled Machine for Heat Fusing Polyolefin Pipes. In all of these machines, the pipe surfaces to be joined are brought to a molten state using a heater which is substantially planar in nature. That is, since the pipes are being joined end-to-end at planar faces, the heater inserted between them to simultaneously bring the faces to a molten state is a relatively thin planar structure. The heater is, therefore, relatively lightweight and easy to handle and consequently is suitable for use in field operations.
In the aforementioned U.S. Pat. No. 4,352,708, fusion of a branch pipe saddle fitting to the side wall of a main pipe is also taught. Branch pipe is then fused to the saddle fitting. However, while the machine therein disclosed can be used for sidewall fusion, the heater required to bring the surfaces to be joined on larger diameter pipes to a molten state is comparatively large and unwieldy for use in the field. Since the surfaces to be joined are curved, the heater must have opposed faces with complementary curves. Since these curved surfaces are disposed on opposite sides of a planar thermal element, the heater is much thicker and, in the case of larger diameter pipes, also comparatively larger diameter, so as to require the use of a crane or other lifting device to manipulate the heater. Therefore, sidewall fusion on pipes greater than 8" in diameter cannot be practically accomplished in the field using known heaters. The present practice for larger diameter pipes is to make a T-configuration in the plant using a lifting device such as a crane to manipulate the heater, to ship the T-configuration to the site, to cut the main pipeline and remove a section of the pipe and then to weld the T-configuration to the main pipeline using existing end-to-end fusion technology. Furthermore, the undesirable thickness of the heater results in longer heat-up times for the heater, perhaps as much as half an hour, and poor temperature distribution over the heater surface which in turn causes an undesirable uneven melt in the pipe surface.
It is, therefore, a primary object of this invention to provide a heater usable in the field to bring surfaces of larger diameter pipe to molten condition for sidewall fusion. It is also an object of this invention to provide a heater usable in the sidewall fusion of polyolefin pipes which is lightweight and easy to handle. A further object of this invention is to provide a heater usable in the sidewall fusion of polyolefin pipes which is a relatively thin laminar assembly of a heating element sandwiched between relatively thin surface-contacting plates. Another object of this invention is to provide a heater usable in the sidewall fusion of polyolefin pipes which is adapted to be easily separated from the molten surfaces of the main and branch pipes. It is also an object of this invention to provide a heater usable in the sidewall fusion of polyolefin pipes which is easily removed from its operating position between the molten surfaces of the main and branch pipes. A further object of this invention is to provide a heater usable in the sidewall fusion of polyolefin pipes which has a relatively short heat-up time, perhaps in the order of 12 minutes. Another object of this invention is to provide a heater usable in the sidewall fusion of polyolefin pipes which provides relatively even temperature distribution on the heater contact surfaces.